RouterGod is pleased to bring another in the guest lecture
series, Priority Queuing by the former star of Different Strokes and current
shopping mall security guard, Gary Coleman. Mr. Coleman arrived dressed
very nicely and was a pleasure to work with, unlike other child celebrities
we've interviewed who we won't mention (Danny Bonaduce). At first we
thought someone was playing a prank on us, ringing our doorbell and running off,
but after the sixth time we noticed Gary standing there with an angry
expression. We apologize for the confusion.

Hey! Whatzup? Anyway people, "whatcha talkn
about"? Ha Ha , just had to throw that in. Good golly
miss molly, let's get down to business. I have been asked to come
here and tell you folks about Priority Queuing and how to configure it
on a Cisco Router. Ever notice how they call an Ethernet frame
smaller that 64 bytes a runt? I take umbrage with that. They
should just call it a "little frame", we would get the
picture. You know what I'm say'n? Okey Dokey, priority
queuing...yeah, ok. Nuth'n to it but to do it.

Priority queuing, like all methods of queuing can only be enabled on
interfaces with bandwidths less than 2.048 Mbps. In other words,
you aint gonna configure queuing on an ethernet interface. So it's
for a WAN link, like ISDN or frame relay. Now listen up
people! If your link is already overloaded, queuing won't
help. It's only to prioritize time sensitive traffic over lower
priority traffic. With Priority Queuing, you can have up to 4
queues. They are called: high, medium, normal and low.
Priority queuing is configured using the priority-list command.
The priority list assigns incoming packets to each of the 4 queues based
on arguments you configure.

The high priority queue is always emptied first, then the medium
queue gets emptied. Next the normal queue gets emptied
and finally the low queue gets emptied. If the high queue
is always full and never gets emptied, none of the other queues will
ever get serviced. That sucks! This is why you probably
don't want to use priority queuing, you probably want to use custom
queuing. Oh, one more thing, people, if a lower priority queue is
getting serviced and traffic arrives into a higher priority queue, you
guessed it, the router forgets all about the queue it was servicing and
immediately takes care of the higher priority queue. Now that I
think of it, Hollywood runs the same way.

Now check this out, you had better assign a default queue for traffic
that you forgot to include in the other queues. Use the priority-list
default command. Your boss swore up and down that the only protocol
on the network was IP, but what he doesn't know is there is an IPX
server that has been in QA since 1988. Moral of the story,
configure a default queue. You can prioritize incoming packets
based on protocol or interface. You can even use the
keyword list to call an access list. Hot Damn! Oh,
yeah, one more thing you can use the priority-list queue-limit command
to change the maximum number packets allowed in each queue to begin
with. By default from high to low, it's 20, 40, 60 and 80.
Here's a config I worked out on the way over here: